In 58 BC at Bibracte in modern France, a Roman army under Julius Caesar defeated the Helvetii – a group of Celtic tribes resident in the fertile area between the Alps and the Jura – and forced them to move en masse to the western part of Switzerland to serve as an irregular frontier force. Over the next hundred years or so, after also conquering Rhaetia (modern Graubünden), the Romans gradually opened up the country, building the first roads over the major Alpine passes – most significantly the Grand-St-Bernard, as well as the Julier and Splügen further east – and founding provincial towns at Nyon, Augst near Basel, and Avenches, the last of which became the Roman capital, with more than fifty thousand inhabitants in its heyday. For two centuries or more under the Romans, Switzerland enjoyed peace and prosperity, with towns established at Geneva, Lausanne, Martigny, Baden (a huge spa), Zürich, Chur and elsewhere. Agriculture flourished, and the region’s settlements were populated by a cosmopolitan cultural mix of native Celts and settled Roman officials. The peace was shattered in 260, when the Alemanii – a group of Celtic tribes from the area of modern Germany – broke through the Romans’ fortified northern border and pushed southwards. Amidst increasing turmoil and confusion, embattled Roman Helvetia and Rhaetia were reduced to impoverished frontier regions.